Writer with Laterally Graded Spin Layer MsT

ABSTRACT

A spin transfer torque reversal assisted magnetic recording (STRAMR) writer is disclosed wherein a spin torque oscillator has a flux guiding layer (FGL) wherein magnetization flips to a direction substantially opposing the write gap (WG) field when sufficient current (IB) density is applied across the STO between a trailing shield and main pole (MP) thereby enhancing the MP write field. A key feature is that the FGL has a center portion with a larger magnetization saturation×thickness (MsT) than in FGL outer portions proximate to STO sidewalls. Accordingly, lower IB density is necessary to provide a given amount of FGL magnetization flipping and there is reduced write bubble fringing compared with writers having a FGL with uniform MsT. Lower MsT is achieved by partially oxidizing FGL outer portions. In some embodiments, there is a gradient in outer FGL portions where MsT increases with increasing distance from FGL sidewalls.

RELATED PATENT APPLICATIONS

This application is related to the following: U.S. Pat. Nos. 10,446,178;10,490,216; U.S. application Ser. No. 16/190,790, filed on Nov. 14,2018; and U.S. application Ser. No. 16/372,517, filed on Apr. 2, 2019;assigned to a common assignee, and herein incorporated by reference intheir entirety.

TECHNICAL FIELD

The present disclosure relates to a spin torque oscillator (STO) alsoknown as a spin flipping element in a write gap (WG) of a spin torquemagnetization reversal assisted magnetic recording (STRAMR) writerwherein the STO is comprised of a magnetic flux guiding layer (FGL)sandwiched between a spin preserving layer and a non-spin preservinglayer, and having a higher saturation magnetization×thickness (MsT)value in a FGL center portion than in adjoining FGL outer portions toenable easier flipping and reduce bubble fringing, and wherein the FGLmagnetic moment flips to an opposite direction of the write gap (WG)field when a current (I_(B)) of sufficient magnitude is applied acrossthe STO during a write process thereby increasing the reluctance in theWG and forcing additional flux out of the main pole (MP) tip at the airbearing surface (ABS) to enhance the write field on the magneticrecording medium.

BACKGROUND

As the data areal density in hard disk drive (HDD) writing increases,write heads and media bits are both required to be made in smallersizes. However, as the write head size shrinks, its writabilitydegrades. To improve writability, new technology is being developed thatassists writing to a media bit. One approach that is currently beinginvestigated is microwave assisted magnetic recording (MAMR), which isdescribed by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”,IEEE Trans. Magn., vol. 44, pp. 125-131 (2008).

In a MAMR writer, the main pole generates a large local magnetic fieldto change the magnetization direction of the medium in proximity to thewriter. By switching the direction of the field using a switchingcurrent that drives the writer, one can write a plurality of media bitson a magnetic recording medium. In MAMR, a spin torque oscillator (STO)is inserted in the WG, and when a critical current is applied, a STOoscillation layer is driven into a precessional state to apply a RFfield on a magnetic medium bit to provide a MAMR assist by lowering bitcoercivity and thereby lower the switching current necessary to providea MP field for a write process. Magnetic flux in the main pole proceedsthrough the ABS and into a medium bit layer and soft underlayer (SUL).In some common designs, the flux returns to the write head through atrailing side loop comprised of a trailing shield structure, and througha leading side loop that includes a leading shield and back gapconnection. There is also a gap field that exits the main pole throughthe write gap, side gaps, and leading gap, and is not directlyresponsible for writing.

Although MAMR has been in development for a number of years, it has notshown enough promise to be introduced into any products yet because ofseveral technical problems. One problem is a fringing growth when thespin torque oscillator (STO) bias is turned on to provide a STRAMRassist. Thus, in addition to a MAMR assist at a relatively low appliedcurrent density, the oscillation layer (FGL) magnetization may flip tobe anti-parallel to the WG field at a higher applied current density. Asa result, the reluctance in the WG is increased thereby boosting the MPwrite field and the return field to the trailing shield. To counteractthe tendency of a growth in fringing as the MP write field increases, arecessed STO has been proposed and is described in related U.S. Pat. No.10,446,178.

Spin transfer (spin torque) devices are based on a spin-transfer effectthat arises from the spin dependent electron transport properties offerromagnetic (FM)-non-FM spacer-FM multilayers. When a spin-polarizedcurrent passes through a magnetic multilayer in a CPP (currentperpendicular to plane) configuration, the spin angular moment ofelectrons from a first FM layer (FM1) that are incident on a second FMlayer (FM2) interacts with magnetic moments of FM2 near the interfacebetween the FM2 and non-FM spacer. Through this interaction, theelectrons transfer a portion of their angular momentum to FM2 (i.e.FGL). As a result, spin-polarized current can switch the FM2magnetization direction if the current density is sufficiently high.

Existing MAMR designs utilize a STO device in one or more of the writegap, leading gap, and side gaps adjoining the MP that produce amagnetization after spin flipping that substantially opposes a field inthe WG, leading gap, and side gaps, respectively. Although a STO havinga greater width and made of a higher Ms (saturation magnetization)material can generate a larger assist effect than a lower Ms material,the STO with the higher Ms has a FGL that is more difficult to flip.Thus, a larger applied current density is needed for FGL magnetizationflipping that induces larger write bubble fringing from the STO edgecorners. Accordingly, an improved STO is needed where a given amount ofwrite assist (FGL flipping) is provided with a relatively low appliedcurrent density, and causes less write bubble fringing than withexisting designs.

SUMMARY

One objective of the present disclosure is to provide a STO device in awrite gap of a MAMR writer wherein FGL magnetization flipping isenhanced at a given applied current density across the STO device toprovide a substantial STRAMR assist and avoiding an undesirable increasein write bubble fringing.

A second objective of the present disclosure is to provide a process offorming a STO device according to the first objective wherein theprocess flow uses existing methods and materials.

According to the one embodiment of the present disclosure, theseobjectives are achieved with a STO device having a FGL sandwichedbetween a non-spin preserving layer (pxL), and a spin preservingconductor layer (ppL). In the exemplary embodiment, the STO device isformed in a WG and the pxL adjoins a trailing side of the MP while theppL contacts a side of the trailing shield (TS) that faces the MPtrailing side. The FGL has a magnetization aligned in the direction ofthe WG field in the absence of an applied current, but oscillates with acone angle to generate a MAMR assist when a current at a first magnitudeis applied from the TS across the STO to the MP. When the appliedcurrent (I_(B)) reaches sufficient magnitude, FGL magnetization flips toan opposite direction with another cone angle that substantially opposesthe WG field and generates a STRAMR assist. Accordingly, there is morereluctance in the WG, which drives more magnetic flux from the MP tip tothe ABS and into a magnetic medium for improved writability. A keyfeature is the FGL has a center portion having a cross-track width of 5nm to 50 nm, and with a substantially greater MsT than outer FGLportions that extend from each side of the FGL center portion to a STOsidewall. Therefore, FGL magnetization may have a greater degree offlipping at a given I_(B) current density than in a conventional FGLwherein there is a uniform MsT. The advantage of the STO of the presentdisclosure is less MP (bubble) fringing because of a reduced I_(B)necessary for a given degree of FGL flipping so that tracks per squareinch (TPI) capability for the writer increases significantly comparedwith prior art MAMR writers.

In a preferred design, the reduced MsT in the outer FGL portion isachieved by performing a natural oxidation (NOX) process after the STOis patterned in the cross-track direction. Thus, a photoresist mask isused to determine STO width during an etch process that removesunprotected regions of STO layers, and remains in place during asubsequent NOX process where oxygen diffuses through the FGL sidewallsand towards a center of the FGL. NOX conditions are controlled so thatthe FGL is not oxidized in the FGL center portion. There may be anoxidation gradient (and MsT gradient) where the oxygen content decreasesand MsT increases with increasing distance from each FGL sidewall untilreaching a minimum and maximum value, respectively, at an interfacebetween each outer oxidized FGL portion and the center FGL portion.Thereafter, a dielectric material is deposited to form the WG, and thephotoresist mask is removed before the TS is deposited and overlyinglayers in the write head are formed.

In a preferred embodiment, the STO has a cross-track width that is atleast 10 nm, but not more than a maximum width of the MP trailing sideat the ABS. The STO has a height of 10 nm to 500 nm that represents adistance (orthogonal to the ABS) between the front side and backside,and a down-track thickness of at least 1 nm. The FGL is one or morelayers of Ni_(x)Fe_(100-x), Co_(y)Fe_(100-y), Co_(z)Ni_(100-z), oralloys thereof, and where x, y, and z are from 0 to 100 atomic %. Thenon-spin preserving layer may be one or more of Ta, Ru, W, Pt, or Tiwhile the spin preserving layer is one of Cu, Ag, Au, Cr, and Al, oralloys thereof.

In an alternative embodiment, the FGL is between first and secondnon-magnetic layers (NM1 and NM2), and there is a spin polarization (SP)layer adjoining NM2 to give a NM1/FGL/NM2/SP STO configuration where theSP layer contacts the TS, and NM1 is on the MP trailing side. In thiscase, the SP layer applies spin torque to the FGL when I_(B) is appliedfrom the TS to the MP and thereby flips FGL magnetization to a directionsubstantially opposite to the WG field when I_(B) has sufficient currentdensity. FGL composition is maintained from the first embodiment whereoxidized outer portions with lower MsT adjoin a higher MsT centerportion that is unoxidized.

The present disclosure also encompasses other STO configurations with akey feature being where the outer portions of the FGL have a lower MsTthan a FGL center portion, preferably by way of an oxidation processbefore depositing the WG.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a head arm assembly of the presentdisclosure.

FIG. 2 is side view of a head stack assembly of the present disclosure.

FIG. 3 is a plan view of a magnetic recording apparatus of the presentdisclosure.

FIG. 4 is a down-track cross-sectional view of a combined read-writehead with leading and trailing loop pathways for magnetic flux return tothe main pole (MP) according to an embodiment of the present disclosure.

FIG. 5A shows an ABS view of a STO device formed in a WG and having aFGL above a MP trailing side, and FIG. 5B is an enlarged view of the STOin FIG. 5A and depicts the FGL with outer FGL portions of width w2 and acenter FGL portion of width w1 according to an embodiment of the presentdisclosure.

FIG. 6 is a down-track cross-sectional view of the writer structure inFIG. 5A where the STO has a front side at the ABS and is between the MPtrailing side and a first trailing shield.

FIG. 7A is a down-track cross-sectional view of the STO in FIGS. 5A-5Bthat shows FGL magnetization substantially parallel to the WG field inthe absence of an applied current, and FIG. 7B indicates that FGLmagnetization flips when a current of sufficient density is appliedacross the STO from the first TS to the MP.

FIG. 7C illustrates FGL magnetization at a first cone angle α, whichflips to a FGL magnetization with cone angle β when applied currentI_(B) has sufficient density according to an embodiment of the presentdisclosure.

FIG. 8 shows an ABS view of a STO having four layers including a FGL,and formed in a WG according to another embodiment of the presentdisclosure.

FIG. 9A is a down-track cross-sectional view of the STO in FIG. 8 thatshows FGL magnetization substantially parallel to the WG field in theabsence of an applied current, and FIG. 9B depicts FGL magnetizationflipping when a current of sufficient density is applied across the STOfrom the first TS to the MP.

FIG. 10 illustrates results of a FEM simulation in the form of adown-track profile of the writer field and indicates a higher writerfield and better field gradient are observed for a writer with a STOaccording to an embodiment of the disclosure than for a writer with aconventional STO.

FIG. 11 depicts an ABS contour plot for the writers in FIG. 10 where thewriter field is set at 5000 Oersted (Oe), and projects EWAC fringingwhere EWAC is erase width in an alternating current (AC) mode.

FIGS. 12-14 are ABS views showing a sequence of steps for forming a STOwith a partially oxidized FGL, and where the STO sidewalls adjoin a WGaccording to an embodiment of the present disclosure.

DETAILED DESCRIPTION

The present disclosure is a writer structure wherein a STO device thatenables both of a STRAMR assist from FGL magnetization flipping, and aMAMR assist for writing on adjacent magnetic bits in a magnetic medium,is formed between a main pole and a trailing shield. The FGL has acenter portion with a greater MsT than in FGL outer portions. In thedrawings, the y-axis is in a cross-track direction, the z-axis is in adown-track direction, and the x-axis is in a direction orthogonal to theABS and towards a back end of the writer structure. Thickness refers toa down-track distance, width is a cross-track distance, and height is adistance from the ABS in the x-axis direction. In some of the drawings,a magnetic bit is considerably enlarged over actual size in order tomore easily depict a magnetization therein. The term “higher degree offlipping” means that FGL magnetization is flipped closer to a directionthat is pointing to the MP trailing side (anti-parallel to the WGmagnetic field). The terms STO, STO device, and STO structure may beused interchangeably. Also, the terms density and magnitude may be usedinterchangeably when referring to applied current that flips FGLmagnetization.

Referring to FIG. 1, a head gimbal assembly (HGA) 100 includes amagnetic recording head 101 comprised of a slider and a PMR writerstructure formed thereon, and a suspension 103 that elastically supportsthe magnetic recording head. The suspension has a plate spring-like loadbeam 222 formed with stainless steel, a flexure 104 provided at one endportion of the load beam, and a base plate 224 provided at the other endportion of the load beam. The slider portion of the magnetic recordinghead is joined to the flexure, which gives an appropriate degree offreedom to the magnetic recording head. A gimbal part (not shown) formaintaining a posture of the magnetic recording head at a steady levelis provided in a portion of the flexure to which the slider is mounted.

HGA 100 is mounted on an arm 230 formed in the head arm assembly 103.The arm moves the magnetic recording head 101 in the cross-trackdirection y of the magnetic recording medium 140. One end of the arm ismounted on base plate 224. A coil 231 that is a portion of a voice coilmotor is mounted on the other end of the arm. A bearing part 233 isprovided in the intermediate portion of arm 230. The arm is rotatablysupported using a shaft 234 mounted to the bearing part 233. The arm 230and the voice coil motor that drives the arm configure an actuator.

Next, a side view of a head stack assembly (FIG. 2) and a plan view of amagnetic recording apparatus (FIG. 3) wherein the magnetic recordinghead 101 is incorporated are depicted. The head stack assembly 250 is amember to which a plurality of HGAs (HGA 100-1 and second HGA 100-2 areat outer positions while HGA 100-3 and HGA 100-4 are at inner positions)is mounted to arms 230-1, 230-2, respectively, on carriage 251. A HGA ismounted on each arm at intervals so as to be aligned in theperpendicular direction (orthogonal to magnetic medium 140). The coilportion (231 in FIG. 1) of the voice coil motor is mounted at theopposite side of each arm in carriage 251. The voice coil motor has apermanent magnet 263 arranged at an opposite position across the coil231.

With reference to FIG. 3, the head stack assembly 250 is incorporated ina magnetic recording apparatus 260. The magnetic recording apparatus hasa plurality of magnetic media 140 mounted to spindle motor 261. Forevery magnetic recording medium, there are two magnetic recording headsarranged opposite one another across the magnetic recording medium. Thehead stack assembly and actuator except for the magnetic recording heads101 correspond to a positioning device, and support the magneticrecording heads, and position the magnetic recording heads relative tothe magnetic recording medium. The magnetic recording heads are moved ina cross-track of the magnetic recording medium by the actuator. Themagnetic recording head records information into the magnetic recordingmedia with a PMR writer element (not shown) and reproduces theinformation recorded in the magnetic recording media by amagnetoresistive (MR) sensor element (not shown).

Referring to FIG. 4, magnetic recording head 101 comprises a combinedread-write head. The down-track cross-sectional view is taken along acenter plane (44-44 in FIG. 5A) formed orthogonal to the ABS 30-30, andthat bisects the MP 14. The read head is formed on a substrate 1 thatmay be comprised of AITiC (alumina+TiC) with an overlying insulationlayer 2 that is made of a dielectric material such as alumina. Thesubstrate is typically part of a slider formed in an array of sliders ona wafer. After the combined read head/write head is fabricated, thewafer is sliced to form rows of sliders. Each row is typically lapped toafford an ABS before dicing to fabricate individual sliders that areused in a magnetic recording device. A bottom shield 4 is formed oninsulation layer 2.

A magnetoresistive (MR) element also known as MR sensor 6 is formed onbottom shield 4 at the ABS 30-30 and typically includes a plurality oflayers (not shown) including a tunnel barrier formed between a pinnedlayer and a free layer where the free layer has a magnetization (notshown) that rotates in the presence of an applied magnetic field to aposition that is parallel or antiparallel to the pinned layermagnetization. Insulation layer 5 adjoins the backside of the MR sensor,and insulation layer 3 contacts the backsides of the bottom shield andtop shield 7. The top shield is formed on the MR sensor. An insulationlayer 8 and a top shield (S2B) layer 9 are sequentially formed on thetop magnetic shield. Note that the S2B layer 9 may serve as a fluxreturn path (RTP) in the write head portion of the combined read/writehead. Thus, the portion of the combined read/write head structure formedbelow layer 9 in FIG. 4 is typically considered as the read head. Inother embodiments (not shown), the read head may have a dual readerdesign with two MR sensors, or a multiple reader design with multiple MRsensors.

The present disclosure anticipates that various configurations of awrite head may be employed with the read head portion. In the exemplaryembodiment, magnetic flux 70 in MP 14 is generated with flowing acurrent through bucking coil 60 a-c and driving coil 61 a-c that arebelow and above the MP, respectively, and are configured in a 1+1Tdesign. The bucking coil and driving coil each have a front portion 60 aand 61 a, respectively, middle portion 60 c and 61 c, respectively, thatare connected through interconnect 51, and each have back portions 60 band 61 b, respectively, that are each connected to a writer pad (notshown).

Magnetic flux 70 exits the MP at pole tip 14 p at the ABS 30-30 and isused to write a plurality of bits on magnetic media 140. Magnetic flux70 b returns to the MP through a trailing loop comprised of trailingshields 17, 18, uppermost (PP3) trailing shield 26, and top yoke 18 x.There is also a leading return loop for magnetic flux 70 a that includesleading shield 11, leading shield connector (LSC) 33, S2 connector (S2C)32, return path (RTP) 9, and back gap connection (BGC) 52. The magneticcore may also comprise a bottom yoke 35 below the MP. Dielectric layers10, 13, 37-39, 42, 43, and 45 are employed as insulation layers aroundmagnetic and electrical components. A protection layer 27 covers the PP3TS and is made of an insulating material such as alumina. Above theprotection layer and recessed a certain distance u from the ABS 30-30 isan optional cover layer 29 that is preferably comprised of a lowcoefficient of thermal expansion (CTE) material such as SiC. Overcoatlayer 28 is formed as the uppermost layer in the write head. In otherembodiments (not shown), the leading return loop is shortened with theremoval of the BGC, or by removing the BGC, RTP, S2C, and LSC to forcemore return flux 70 b through the trailing loop.

Referring to FIG. 5A, there is a MP with MP tip 14 p having track widthTW, trailing side 14 t 1, leading side 14 b 1, and two sides 14 s formedequidistant from a center plane 44-44, and with an all wrap around (AWA)shield structure that was described in related U.S. Pat. No. 10,446,178.However, other shield structures may also be used with the STO deviceembodiments described herein. There is a write gap (WG) 16 withthickness t on the MP trailing side, side gaps 15 adjoining each MP side14 s, and a leading gap 13 below the MP leading side. The trailingshield structure comprises a first TS 17 with a saturation magnetization(Ms) value from 19 kiloGauss (kG) to 24 kG that is formed on the WG. TheTS structure also includes a second TS 18 also known as the write shield(WS) formed on the first TS top surface 17 t and sides 17 s, on WG sides16 s, and on a top surface of the side shields at plane 41-41. Plane41-41 includes the MP trailing side at the ABS. Side shields contact atop surface of the leading shield 11 at plane 46-46 that is parallel toplane 41-41, and includes the MP leading side at the ABS. The writer isshown with outer sides 90, 91.

STO device 22 features a lower non-spin preserving layer (pxL) 21 on MPtrailing side 14 t 1, a middle flux guiding layer (FGL) 20, and an upperspin preserving layer (ppL) 19. The pxL is a single layer or multilayerthat is typically one or more of Ta, W, Pt, Ru, Ti, or Pd so that spinpolarized electrons transiting the pxL will have their spin polarizationrandomized by spin flipping scattering. Moreover, the pxL issufficiently thick so that the MP and FGL are not magnetically coupled.The ppL is a conductive layer and is preferably comprised of Cu, Ag, Au,Al, or Cr, or an alloy thereof in which electrons in applied currentI_(B) (FIG. 7B) will substantially retain their spin polarization whentraversing the ppL.

In the exemplary embodiment, STO width w is essentially equivalent tothe track width of the MP trailing side 14 t 1 at plane 41-41. However,in other embodiments (not shown), width w may be less than the MP trackwidth. Preferably, STO width is at least 10 nm.

Referring to FIG. 5B that is an enlargement of the STO in FIG. 5A, FGL20 is a magnetic layer that is preferably comprised of one or morelayers of Ni_(x)Fe_(100-x), Co_(y)Fe_(100-y), Co_(z)Ni_(100-z), andwhere x, y, and z are from 0 atomic % to 100 atomic %, or alloys thereofwith one or more additional elements. The FGL has a center portion 20 chaving width w1 of 5 nm to 50 nm, and a first MsT (MsT₁) from 1 nmT(nm×Tesla product) to 14 nmT, and an outer portion 20 x having a secondMsT (MsT₂) and adjoining each side of the FGL center portion. Each FGLouter portion has width w2 of 5 nm to 20 nm, and a MsT₂ from 1 nmT to 8nmT and where MsT₂<MsT₁. Furthermore, MsT₂ may have a gradient whereMsT₂ increases continuously with increasing distance from STO sidewall20 s until reaching an interface with the FGL center portion. In anotherembodiment, MsT₂ is substantially uniform across each FGL outer portion.Note that the sum (2w2+w1)=w.

As described in a later section with regard to FIGS. 12-14, onepreferred method of forming FGL outer portions wherein MsT₂ is less thanMsT₁ in the FGL center portion is to perform a natural oxidation (NOX)process after the STO device is patterned in the cross-track directionto form sidewall 20 s on each side of center plane 44-44. Thus,oxidation conditions may be controlled to give a limited oxygendiffusion length into the FGL.

In FIG. 6, the down-track cross-sectional view at center plane 44-44 inFIG. 5A is illustrated and shows a portion of the writer that isproximate to MP tip 14 p at the ABS 30-30 according to an embodiment ofthe present disclosure. MP leading side 14 b 1 is tapered and extendsfrom the ABS 30-30 to MP bottom surface 14 b 2 that is alignedorthogonal to the ABS. Moreover, top surface 11 t of the leading shield11 is substantially parallel to the tapered MP leading side andseparated therefrom by leading gap 13. MP trailing side 14 t 1 is alsotapered, and connects at corner 14 c with MP top surface 14 t 2 that isparallel to the MP bottom surface. In other embodiments (not shown), oneor both of the MP leading and trailing sides that end at the ABS may bealigned orthogonal to the ABS and form a continuous planar surface withMP bottom and top surfaces, respectively. WG 16 is formed between MPtrailing side 14 t 1 and a first TS front portion 17. The first TS alsohas a back portion 17 e that is parallel to MP top surface 14 t 2, andis separated therefrom by the WG and dielectric layer 42 formed on theMP top surface. STO 22 has a front side 22 f at the ABS and contacts thefirst TS at side 17 b. In an alternative embodiment (not shown), the STOfront side is recessed 2 nm to 100 nm from the ABS, and separatedtherefrom by the WG.

In FIG. 7A, STO 22 in FIG. 6 is enlarged to show pxL 21 formed on MPtrailing side 14 t 1, and ppL 19 adjoining first TS 17. The writeprocess is shown in an example where MP field 70 is pointing down (outof MP 14) in order to write a magnetic bit 99 with magnetization 99 mpointing up in bit layer 142 on a soft underlayer 141 in magnetic medium140. Return field 70 b enters the trailing loop for magnetic flux returnto the MP at first TS 17. The MP has a local magnetization 14 m at theMP trialing side that is aligned substantially in the same direction asWG field H_(WG) that proceeds from the MP to the first TS. Moreover, FGL20 has magnetization 20 m where both magnetizations 17 m and 20 m aresubstantially aligned with H_(WG) in the absence of an applied currentacross the STO.

FIG. 7B shows that the writer of the present disclosure is capable ofboth of a MAMR assist and a spin torque reversal assisted magneticrecording (STRAMR) assist in the write process. In particular, atcertain current densities for applied current I_(B), magnetization 20 mmaintains a direction substantially parallel to H_(WG) and has aprecessional state 20 p with cone angle α as depicted in FIG. 7C. AsI_(B) current density increases, cone angle α increases untilmagnetization 20 m flips to precessional state 20 p′ where magnetization20 m now has cone angle β pointing substantially opposite to H_(WG). Inboth precessional states, RF field 77 is generated and directed at bitlayer 142 and lowers the write field necessary to switch bitmagnetization 99 m to provide a MAMR assist. However, a STRAMR assist isalso present with precessional state 20 p′ and increases as cone angle βdecreases to 0 degrees. The STRAMR assist results from increasedreluctance in WG 16 and forces more magnetic flux 70 exit MP 14 as writefield 70. In other words, only a MAMR assist is possible withprecessional state 20 p, but both of a MAMR and STRAMR assist may occurin precessional state 20 p′.

Current I_(B) is applied from a direct current (DC) source 50 throughlead 58 and first TS 17, and across STO 22 from first TS side 17 b to MPtrailing side 14 t 1, and exits MP 14 through a second lead 57. Notethat the flow of electrons is opposite to the I_(B) direction and isfrom the MP to the first TS. It should be understood that the electricalcurrent (I_(B)) direction required for the FGL to provide a STRAMRassist is from TS shield→spin preserving layer→FGL→non-spin preservinglayer→MP. Furthermore, the I_(B) direction is independent of the gapfield direction. Thus, the I_(B) direction stays the same when the writefield (and H_(WG)) is switched to the opposite direction in order towrite a transition.

STO device 22 is configured so that sufficient spin torque (not shown)is exerted on FGL 20 (from backscattered electrons from the first TS) toflip the FGL magnetization. The flipping mechanism is based on thebehavior of electrons with spins parallel and anti-parallel to themoment in the first TS. The portion of electrons having a moment that isparallel to TS magnetization 17 m is able to enter first TS 17 with verylittle resistance. However, electrons with a moment that isanti-parallel to first TS magnetization proximate to side 17 b do notenter the first TS easily because of less unoccupied states in the firstTS, and are backscattered to the FGL. As a result, spin torque isexerted on FGL magnetization 20 m, and the FGL magnetization is flippedto a direction primarily oriented toward MP trailing side 14 t 1.

The degree of FGL magnetization flipping is determined by the magnitudeof I_(B) current density. A higher degree of flipping means that coneangle β is smaller and provides a greater STRAMR assist (lower MAMRassist) than at a lower I_(B) current density that gives a lower degreeof flipping. Improved STO devices are desired where a lower I_(B)current density is required to provide a given amount (degree) of FGLmagnetization flipping so that an improved STRAMR assist is realizedwith a minimum amount of write bubble fringing. Accordingly, there willbe less STO device heating (better stability) and better EWACperformance while maintaining high TPI capability. This objective isachieved in STO 22 because outer FGL portions have a lower MsT than inthe FGL center portion (MsT₂<MsT₁) thereby allowing FGL magnetizationflipping at a lower I_(B) current density than in the prior art wherethe FGL has a uniform MsT throughout the layer.

According to another embodiment of the present disclosure illustrated inFIG. 8, STO 22 of the first embodiment may be replaced with STO 22-1having a first non-magnetic spacer (NM1) 23, FGL 20, second non-magneticspacer (NM2) 24, and spin polarization (SP) layer 25 sequentially formedon MP trailing side 14 t 1, and where the SP layer contacts first TS 17at side 17 b. This STO design was previously disclosed in related U.S.application Ser. No. 16/372,517. The SP layer may be comprised of one ofthe magnetic materials mentioned previously with respect to FGL 20, andgenerates spin torque on the FGL in the presence of I_(B) withsufficient current density to cause FGL magnetization flipping toprecessional state 20 p′ with cone angle β (FIG. 7C). NM1 and NM2 may bea single layer or multilayer films, and are preferably a non-magneticmetal with a long spin diffusion length such as Cu, Ag, or Au that serveas a spin preserving layer so that electrons spin polarized by the SPlayer 25 do not encounter strong spin-flip scattering in the spacers. Akey feature is that the FGL has outer portions 20 x with a lower MsTthan that in FGL center portion 20 c similar to the FGL structure inFIG. 5B.

In FIG. 9A, STO 22-1 is formed in WG 16 between MP trailing side 14 t 1and first TS 17. Similar to the first embodiment, FGL magnetization isaligned substantially in the same direction as local MP magnetization 14m, local first TS magnetization 17 m, and WG field H_(WG) in the absenceof applied current across the STO. Moreover, SP layer 25 hasmagnetization 25 m that is ferromagnetically coupled with first TSmagnetization 17 m and is pointing toward the first TS.

Referring to FIG. 9B, when current I_(B) with sufficient current densityis applied from the first TS at first TS side 17 b across STO 22-1 tothe MP at MP trailing side 14 t 1, FGL magnetization flips to adirection substantially opposing the WG field. As in the previousembodiment, there is an advantage over the prior art in that the FGLflipping occurs at lower I_(B) current density and bubble fringing isminimized because of lower MsT in outer FGL portions 20 x than in FGLcenter portion 20 c (MsT₂<MsT₁).

The present disclosure also anticipates that other STO configurationsmay be employed rather than STO 22 and STO 22-1 described previously.For example, in related U.S. Pat. No. 10,490,216, a STO is disclosedwhere two spin polarization layers apply spin torque to a FGL fromopposite sides. The spin torques are additive and create a larger spintorque than achieved with a single SP layer so that the I_(B) currentdensity is reduced for FGL magnetization flipping, or there is a greaterFGL magnetization flipping at the same I_(B) current density.

A magneto-static modeling study was performed to compare three writerswith simplified assumptions. Head 1 is a process of record (POR) writerwith a STO where the entire FGL has a MsT of 16 nmT, and assumingmagnetization in the entire FGL is flipped. Head 2 is the POR writerwith the assumption that only the center 20 nm width portion of the FGLis 100% flipped while a 10 nm outer FGL on each side of the centerportion is not flipped at all. Head 3 is a writer according to anembodiment of the present disclosure where a center FGL portion that is20 nm wide has MsT₁=16 nmT, outer FGL portions that are each 10 nm widehave a MsT₂=8 nmT, and magnetization in the entire FGL is flipped.Although the deep gap field is not uniform across the cross-trackdirection (gap field in the center is significantly larger than gapfield off the center), the FGL in the new STO design (Head 3) will besubstantially easier to flip than a FGL in a conventional STO (POR)writer. Thus, the actual behavior of the POR writer will be close toHead 2 and the actual behavior of the STO design in the presentdisclosure will be close to Head 3.

FIG. 10 illustrates that Head 3 (curve 73) has higher writer field andfield gradient compared with Head 2 (curve 72). Head 1 results arerepresented with curve 71. In FIG. 11, Head 3 (curve 73 c) showsessentially the same EWAC fringing as Head 2 (curve 72 c). Meanwhile,Head 1 results are displayed as curve 71 c. In an operating condition Awhere the base writer structure releases a strong field and both PORwriters and the Head 3 design have a FGL that is 100% flipped ((3 coneangle proximate to 0 degrees in precessional state 20 p′ shown in FIG.7C), the Head 3 design will assist center track writing in essentiallythe same magnitude as the POR writers. Head 3 will have an advantageover the POR writers in terms of less fringing and adjacent trackinterference (ATI). With an alternative operating condition B where asmaller I_(B) current density is used and only magnetization in theouter FGL portions in Head 3 is flipped (β cone angle) whilemagnetization in the center FGL portion is not flipped (still cone angleα), which is effectively less than 100% FGL flipping, a stray field isinduced in the side shields (SS) but the stray field is significantlysmaller in the SS adjacent to the MP in Head 3 than in the SS adjacentto the MP in Head 2 (or Head 1). Accordingly, the Head 3 designeffectively reduces the SS stray field as a result of having a lower MsTin the FGL outer portions than in a FGL center portion. Therefore, thewriter with the STO according to the present disclosure is expected toprovide better performance than a POR writer with a conventional STO ineither of condition A (100% FGL flipping) or condition B (<100% FGLflipping).

The present disclosure also encompasses a process sequence forfabricating a STO comprised of a FGL having outer portions with a MsTless than a MsT in a FGL center portion. According to one embodiment ofthe present disclosure depicted in FIGS. 12-14, the feature where FGLouter portions have MsT₂, and FGL center portion has MsT₁ whereMsT₂<MsT₁, is produced by employing a NOX process to partially oxidizethe FGL after the STO is patterned in the cross-track direction.

Referring to FIG. 12, the partially formed writer structure including MPtip 14 p that adjoins side gaps 15 and leading gap 13 is providedaccording to a conventional process sequence that is not describedherein. Each side shield 12 has a top surface 12 t that is coplanar witha trailing edge of the MP tapered trailing side 14 t 1 at plane 41-41,which is orthogonal to the subsequently formed ABS plane. In theexemplary embodiment, STO layers comprised of pxL 21, FGL 20, and ppL 19are sequentially deposited on SS top surfaces, side gaps, and on the MPtrailing side. Thereafter, a photoresist mask 58 with sides 58 s thatare separated by cross-track width w is formed on ppL 19, and above theMP trailing side, using a photolithography method well known in the art.Then, an ion beam etch (IBE) or reactive ion etch (RIE) 110 is performedto remove unprotected regions of the STO layers and stops on the SS topsurfaces and side gaps. As a result, STO sidewalls including FGLsidewalls 20 s are formed, and the FGL has a width substantially equalto w.

Referring to FIG. 13, a key step in the FGL formation process is a NOXstep 111 that is used to partially oxidize FGL 20 to give oxidized outerFGL portions 20 x having width w2, and an unoxidized center FGL portion20 c of width w1 described earlier and shown in FIG. 5B. NOX conditionsare known in the art and are not described herein. There may be acontinuous gradient in the extent of oxidation (and MsT₂) across eachFGL outer portion such that the oxygen content decreases (and MsT₂increases) with increasing distance from each FGL sidewall 20 s untilreaching an interface with the FGL center portion at a distance w2 froma FGL sidewall. Preferably, w1 is from 5 nm to 50 nm. As indicatedearlier, MsT₁ of the FGL center portion is from 1-14 nmT while the MsT₂in FGL outer portions is in the range of 1-8 nmT, and where MsT₂<MsT₁.However, the present disclosure anticipates that FGL outer portions mayalso be formed with a process that generates a substantially uniformMsT₂ therein.

In FIG. 14, the partially formed writer is shown after WG 16 isdeposited on SS top surfaces 12 t, side gaps 15, and adjoining STOlayers 19-21. A chemical mechanical polish (CMP) process may beperformed to remove the photoresist mask and yield ppL top surface 19 tthat is coplanar with WG top surface 16 t. Thereafter, conventionalmethods are employed to form the remainder of the write head in thewriter structure, and then an ABS is typically formed with a lappingprocess.

While the present disclosure has been particularly shown and describedwith reference to, the preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made without departing from the spirit and scope of thisdisclosure.

1. A spin torque magnetization reversal assisted magnetic recording(STRAMR) writer, comprising: (a) a main pole (MP) comprised of atrailing side having a track width w at an air bearing surface (ABS);(b) a write gap (WG) formed on the MP trailing side; (c) a firsttrailing shield (TS) on the WG and having a first side facing the MP;and (d) a spin torque oscillator (STO) formed in the WG between the MPtrailing side and first side of the first TS, and comprising a fluxguiding layer (FGL) with a magnetization aligned substantially parallelto a WG field (H_(WG)) in the absence of an applied current across theSTO, and wherein the STRAMR writer is configured so that the FGLmagnetization flips to a direction substantially opposing H_(WG) when acurrent I_(B) of sufficient magnitude is applied from the first TSacross the STO to the MP, the FGL comprises: (1) a center portion havinga first saturation magnetization×thickness product (MsT₁) and a firstcross-track width w1 between two sides thereof; and (2) an outer portionadjoining each side of the FGL center portion and having a secondcross-track width w2, and a second saturation magnetization×thicknessproduct (MsT₂) where MsT₂<MsT₁.
 2. The STRAMR writer of claim 1 whereina sum (2w2+w1) is less than or equal to the track width of the MPtrailing side.
 3. The STRAMR writer of claim 1 wherein MsT₁ is from 1nm×Tesla (nmT) to 14 nmT.
 4. The STRAMR writer of claim 1 wherein w1 isfrom about 5 nm to 50 nm, and w2 is from about 5 nm to 20 nm.
 5. TheSTRAMR writer of claim 1 wherein MsT₂ is from 1 nmT to 8 nmT.
 6. TheSTRAMR writer of claim 1 wherein the FGL center portion is an unoxidizedmagnetic layer, and each FGL outer portion is an oxidized magneticlayer.
 7. The STRAMR writer of claim 6 wherein an oxygen content in eachFGL outer portion is a continuous gradient such that the oxygen contentdecreases and MsT₂ increases with increasing distance from a FGLsidewall up to the FGL center portion.
 8. The STRAMR writer of claim 1wherein MsT₂ is substantially uniform within each FGL outer portion. 9.The STRAMR writer of claim 1 wherein the STO further comprises anon-spin preserving layer (pxL) contacting the MP trailing side, and aspin preserving layer (ppL) adjoining the first side of the first TS togive a pxL/FGL/ppL STO configuration.
 10. The STRAMR writer of claim 1wherein the STO further comprises a first non-magnetic spacer (NM1)between the MP trailing side and a first side of the FGL, a secondnon-magnetic spacer (NM2) on a second side of the FGL, and a spinpolarization (SP) layer on the NM2 to give a NM1/FGL/NM2/SP STOconfiguration.
 11. The STRAMR writer of claim 1 wherein the FGLcomprises one or more of layers of Ni_(x)Fe_(100-x), Co_(y)Fe_(100-y),Co_(z)Ni_(100-z), and where x, y, and z are from 0 atomic % to 100atomic %, or alloys thereof with one or more additional elements. 12.The STRAMR writer of claim 9 wherein the pxL is a single layer ormultilayer of one or more of Ta, W, Pt, Ru, Ti, and Pd so that spinpolarized electrons in the applied current I_(B) transiting the pxL willhave a spin polarization randomized by spin flipping scattering.
 13. TheSTRAMR writer of claim 9 wherein the ppL is Cu, Ag, Au, Al, or Cr, or analloy thereof in which electrons in the applied current I_(B) willsubstantially retain a spin polarization when traversing the ppL. 14.The STRAMR writer of claim 1 wherein the STO has a front side that isexposed at the ABS or is recessed behind the ABS to a height of 2 nm to100 nm.
 15. A head gimbal assembly (HGA), comprising: (a) the STRAMRwriter of claim 1; and (b) a suspension that elastically supports theSTRAMR writer, wherein the suspension has a flexure to which the STRAMRwriter is joined, a load beam with one end connected to the flexure, anda base plate connected to the other end of the load beam.
 16. A magneticrecording apparatus, comprising: (a) the HGA of claim 15; (b) a magneticrecording medium positioned opposite to a slider on which the STRAMRwriter is formed; (c) a spindle motor that rotates and drives themagnetic recording medium; and (d) a device that supports the slider,and that positions the slider relative to the magnetic recording medium.17. A method of forming a spin torque magnetization reversal assistedmagnetic recording (STRAMR) writer, comprising: (a) providing a mainpole (MP) with a trailing side having a track width w at a first plane,a side gap adjoining a side of the MP on each side of a center planethat is orthogonal to the first plane and bisects the MP trailing side,and a side shield adjoining a side of each side gap that faces away fromthe center plane; (b) depositing a spin torque oscillator (STO) stack oflayers comprising a flux guiding layer (FGL) that has a switchablemagnetization, the STO stack is formed on the MP trailing side, sidegaps, and side shields; (c) patterning the STO stack of layers in across-track direction such that the FGL has a sidewall on each side ofthe center plane; (d) performing an oxidation process to form FGL outerportions that are oxidized and each having a cross-track width w2, asaturation magnetization×thickness (MsT₂) and bounded by the FGLsidewall on an outer side, and a center unoxidized FGL portion havingcross-track width w1 and a saturation magnetization×thickness (MsT₁)between the FGL outer portions, wherein MsT₁>MsT₂; (e) depositing awrite gap (WG) on the side gaps and side shields, and that adjoins theFGL sidewalls; and (f) forming an air bearing surface (ABS) at the firstplane.
 18. The method of claim 17 wherein a sum (2w2+w1) is less than orequal to the track width of the MP trailing side.
 19. The method ofclaim 17 wherein MsT₁ is from 1 nm×Tesla (nmT) to 14 nmT, and MsT₂ isfrom 1 nmT to 8 nmT.
 20. The method of claim 17 wherein w1 is from about5 nm to 50 nm, and w2 is from about 5 nm to 20 nm.
 21. The method ofclaim 17 wherein an oxygen content in each oxidized FGL outer portion isa continuous gradient such that the oxygen content decreases and MsT₂increases with increasing distance from each FGL sidewall up to thecross-track width w2.
 22. The method of claim 17 wherein MsT₂ and anoxygen content are substantially uniform within each oxidized FGL outerportion.
 23. The method of claim 17 wherein the STO further comprises anon-spin preserving layer (pxL) contacting the MP trailing side, and aspin preserving layer (ppL) adjoining a first side of a first trailingshield (TS) to give a pxL/FGL/ppL STO configuration.
 24. The method ofclaim 17 wherein the STO further comprises a first non-magnetic spacer(NM1) between the MP trailing side and a first side of the FGL, a secondnon-magnetic spacer (NM2) on a second side of the FGL, and a spinpolarization (SP) layer on the NM2 to give a NM1/FGL/NM2/SP STOconfiguration.
 25. The method of claim 17 wherein the FGL comprises oneor more of layers of Ni_(x)Fe_(100-x), Co_(y)Fe_(100-y),Co_(z)Ni_(100-z), and where x, y, and z are from 0 atomic % to 100atomic %, or alloys thereof with one or more additional elements. 26.The method of claim 23 wherein the pxL is a single layer or multilayerthat is one or more of Ta, W, Pt, Ru, Ti, and Pd so that spin polarizedelectrons in an applied current across the STO, and from the first TS tothe MP trailing side will have a spin polarization randomized by spinflipping scattering when transiting the pxL.
 27. The method of claim 23wherein the ppL is Cu, Ag, Au, Al, or Cr, or an alloy thereof in whichelectrons in an applied current across the STO and from the first TS tothe MP trailing side will substantially maintain a spin polarizationwhen traversing the ppL.
 28. The method of claim 17 wherein theoxidation process is a natural oxidation (NOX) process.